The entry of Chlamydia species into host cells in culture and their subsequent fate therein will be studied with respect to the following topics: 1. the molecular structures of the sites on host and parasite surfaces that interact to bring about attachment and ingestion of the parasite, 2. the structural differences between the phagosomal membranes that are responsible for the inhibition of the fusion of lyosomes with chlamydial phagosomes, and 3. the reasons for the difficulty with which non-lymphogranuloma venereum strains of Chlamydia trachomatis (as compared with other chlamydial strains) enter host cells.